Recombinant DNA technology has enabled the cloning and expression of genes encoding medically and agriculturally important proteins and glycoproteins. Such products include, for example, insulin, growth hormone, growth hormone releasing factor, somatostatin, tissue plasminogen activator, tumor necrosis factor, lipocortin, coagulation factors VIII and IX, the interferons, colony stimulating factor, the interleukins and urokinase.
Many of these important proteins, however, are large (molecular weights in excess of 30Kd), secreted, require sulfhydryl bonds to maintain proper folding, are glycosylated and are sensitive to proteases. As a result, the recombinant production of such products in prokaryotic cells has proven to be less than satisfactory because the desired recombinant proteins are incorrectly processed, lack proper glycosylation or are improperly folded. Accordingly, resort has been had to the production of those recombinant proteins in cultured eukaryotic cells. This technique has proven to be both expensive and often unreliable due the variability of cell culture methods. For example, average yields are 10 mg of recombinant protein per liter of culture media, with the resulting cost typically for exceeding $1,000 per gram of recombinant protein. Accordingly, resort has been had to the production of those recombinant proteins in cultured eukaryotic cells.